feat(event_bus): add in-process domain event bus

A standalone, domain-agnostic pub/sub seam: an EventBus that owns its
subscriber registry and streams Event values from producers to listeners
in process. Boundary-crossing (Celery/DB/workers) is left to subscribers,
keeping the bus single-responsibility. Includes the immutable Event value
object and full unit coverage.

surfsense_backend/app/event_bus/__init__.py

@@ -0,0 +1,20 @@
+"""In-process domain event bus.
+
+Domain-agnostic pub/sub. Producers ``await bus.publish(...)``; subscribers
+``bus.subscribe(...)``. Domain modules depend on it, never the reverse.
+
+    from app.event_bus import bus
+    await bus.publish("document.indexed", {"document_id": 42}, search_space_id=7)
+"""
+
+from __future__ import annotations
+
+from .bus import EventBus, Subscriber, bus
+from .event import Event
+
+__all__ = [
+    "Event",
+    "EventBus",
+    "Subscriber",
+    "bus",
+]